Vortex Pinning in Neutron Stars, Slip-stick Dynamics, and the Origin of Spin Glitches

Abstract

We study pinning and unpinning of superfluid vortices in the inner crust of a neutron star using 3-dimensional dynamical simulations. Strong pinning occurs for certain lattice orientations of an idealized, body-centered cubic lattice, and occurs generally in an amorphous or impure nuclear lattice. The pinning force per unit length is $\sim 10^{16}$ dyn cm${}^{-1}$ for a vortex-nucleus interaction that is repulsive, and $\sim 10^{17}$ dyn cm${}^{-1}$ for an attractive interaction. The pinning force is strong enough to account for observed spin jumps (glitches). Vortices forced through the lattice move with a slip-stick character; for a range of superfluid velocities, the vortex can be in either a cold, pinned state or a hot unpinned state, with strong excitation of Kelvin waves on the vortex. This two-state nature of vortex motion sets the stage for large-scale vortex movement that creates an observable spin glitch. We argue that the vortex array is likely to become tangled as a result of repeated unpinnings and repinnings. We conjecture that during a glitch, the Kelvin-wave excitation spreads rapidly along the direction of the mean superfluid vorticity and slower in the direction perpendicular to it, akin to an anisotropic deflagration.

12 pages, 7 figures (two animations)

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• Gapless neutron superfluidity can explain the late time cooling of transiently accreting neutron stars
• Microscopic calculation of the pinning energy of a vortex in the inner crust of a neutron star
• Direct excitation of Kelvin waves on quantized vortices
• Measuring the vortex-nucleus pinning force from pulsar glitch rates
• Searching for continuous gravitational waves from slowly spinning neutron stars with DECIGO, Big Bang Observer, Einstein Telescope and Cosmic Explorer
• Persistent gravitational radiation from glitching pulsars. II. Updated scaling with vortex number
• Vortex Avalanches and Collective Motion in Neutron Stars
• Gapless neutron superfluidity in the crust of the accreting neutron stars KS 1731-260 and MXB 1659-29
• New Molecular Dynamics Methods for Simulating Neutron Star Crusts with Superfluid Vortices